The invention relates to preparing electrets and to enhancing release properties of thermoplastic films using resin additives.
The filtration properties of nonwoven polymeric fibrous webs can be improved by transforming the web into an electret, i.e., a dielectric material exhibiting a quasi-permanent electrical charge. Electrets are effective in enhancing particle capture in aerosol filters. Electrets are useful in a variety of devices including, e.g., air filters, face masks, and respirators, and as electrostatic elements in electro-acoustic devices such as microphones, headphones, and electrostatic recorders.
Electrets are currently produced by a variety of methods including direct current (“DC”) corona charging (see, e.g., U.S. Pat. Re. 30,782 (van Turnhout)), and hydrocharging (see, e.g., U.S. Pat. No. 5,496,507 (Angadjivand et al.)), and can be improved by incorporating fluorochemicals into the melt used to produce the fibers of some electrets (see, e.g., U.S. Pat. No. 5,025,052 (Crater et al.)) and by plasma fluorinating (see, e.g., U.S. Pat. No. 6,397,458 (Jones et al.)).
Many of the particles and contaminants with which electret filters come into contact interfere with the filtering capabilities of the webs. Liquid aerosols, for example, particularly oily aerosols, tend to cause electret filters to lose their electret enhanced filtering efficiency (see, e.g., U.S. Pat. No. 5,411,576 (Jones et al.)). In addition, heat and aging can impair the filter efficiency of some electret filters.
Numerous approaches have been developed to compensate for loss of filtering efficiency of an electret filter. One method for improving an electret filter's efficiency includes increasing the amount of the nonwoven polymeric web in the electret filter by adding layers of web or increasing the thickness of the electret filter. The additional web, however, increases the breathing resistance of the electret filter, adds weight and bulk to the electret filter, and increases the cost of the electret filter.
Another method for improving an electret filter's resistance to oily aerosols includes forming the electret filter from resins that include melt processable fluorochemical additives such as fluorochemical oxazolidinones, fluorochemical piperazines, and perfluorinated alkanes (see, e.g., U.S. Pat. No. 5,025,052 (Crater et al.)), perfluorinated moieties. The fluorochemicals are melt processable, i.e., suffer substantially no degradation under the melt processing conditions used to form the microfibers that are used in the fibrous webs of some electrets (see, e.g., WO 97/07272 (Minnesota Mining and Manufacturing)). Thermally stable organic traizine compounds also have been used to improve charging of an electret filter (see, e.g., U.S. Pat. No. 5,908,598 (Rousseau et al.) and U.S. Pat. No. 6,002,017 (Rousseau et al.)).